It sounds like a great idea, but for my stuff I usually work with local suppliers for prototyping and small production runs. There are inevitably problems in the early stages so it's good to deal with the supplier face-to-face. There's a lot to be said for being able to jump in the car and be at the supplier in a few minutes!

For my third and final act in life (engineer) I am integrating parts of a shop internally into my house. For example dining room is main workroom, other bedroom is going to be wrap around benches with tools/jigs what I'm working on. Yes manufacturing fascinates me. Haha good thing I have a wife that puts up with such things. I could not live without getting my hands dirty.

So while I am not likely to use such a service, I can see how such services could change things like Home Depot changed construction.

I have seen a discussion (don't remember where) in which a lot of machine shop owners were saying that this service wasn't very good for them. They spend a lot of time bidding jobs, but don't get any work unless they bid too low to make money. I like to work with someone that I can talk to, and who will talk to me if they have questions. You can get stuff done on E-machine shop, but I would just use it to find a close place to work with on an ongoing basis.